THIS invention relates to an apparatus for a metal reduction and melting process such as, for example, a steelmaking process, in which a metal and carbon containing burden is heated in a channel type induction furnace in order to reduce and melt the metal containing part of the burden.
The conventional channel type induction furnace usually comprises an elongated tubular heating vessel which is of substantially circular configuration in cross section, and which is heated by two circumferentially spaced, longitudinally disposed, rows of induction heaters, or inductors, of which each row extends along the bottom of the vessel on opposite sides of the longitudinally extending centre line of the vessel.
One such a furnace is, for example, disclosed in U.S. Pat. No. 5,411,570, where it is used for the reduction and melting of a metal and carbon containing burden.
In the aforesaid USA process, the burden is introduced to the heating vessel through two circumferentially spaced apart, longitudinally disposed rows of ports located in the upper wall of the vessel, with the result that the burden floats on the molten metal bath in the vessel as two wedge shaped heaps which each extends along an opposite side of the vessel""s wall, with the wider end of the wedge, i.e. the xe2x80x98apexxe2x80x99 of the heap, being located towards the vessel""s wall, and the narrower end of the wedge, i.e. the xe2x80x98toexe2x80x99 of the heap, towards the middle of the vessel. As a result of this, the apices of the heaps that float on the metal bath are located almost vertically above the inductor throats (openings).
Since the metal is heated by the I2R losses in the inductor, and a convective upward flow of relatively hot metal is caused directly above the inductor throat, more heat reaches the undersides of the heaps in those areas located almost directly under their apices than in other areas. (Hot spots are formed below the highest points of the rows of heaps.) Burden particles are therefore xe2x80x9cconsumedxe2x80x9d mainly in the said areas located almost directly under the apices of the heaps, resulting in a net flow of particles towards these areas.
The flow of burden particles in the heaps can be represented by vectors. Such flow vectors can relate to flow perpendicular to the surface of the heap, and flow parallel thereto. Flow perpendicular to the heap surface is undesirable because heat absorbed as a result of radiation from the roof of the vessel onto the surface can effectively only be conducted to depths in the order of 25 mm. This means that once a particle has travelled approximately 25 mm perpendicular to the surface, it is effectively screened from such radiation. The time required for this movement can be termed the xe2x80x9cexposurexe2x80x9d time.
Reduced exposure times of particles result in reduced radiant energy absorption by these particles. This in turn implies that other particles that reach the toes of the heaps, where the heating rate and hence melting rate caused by the inductors is lower, are exposed to radiation for longer periods than would otherwise be the case. Extended exposure times in turn imply higher surface temperatures and hence reduced radiant heat transfer rates to the material at the toes of the heaps. The relatively high temperatures and high degree of reduction of material at the toes of the heaps may also result in reoxydation because of the lack of protection by reducing gas. (Reduction reactions are completed, hence no CO gas is formed in this region to protect the burden from reoxydation by CO2.
Both over exposure and under exposure of burden particles to radiation are undesirable because of the resultant higher electrical power and reductant consumption.
A further disadvantage found with the aforesaid known arrangement is that the significant difference between the processing of the burden material that reaches the toes of the heaps and the material that does not, results in a significant difference in the ratio of carbon and oxygen available by the time the particles are melted. The relative quantities of these differently constituted materials reaching the liquid bath are greater when high power input rates are applied. When the inhomogeneity so created reaches the stage where the excess carbon dissolved in the one area, and the excess oxygen dissolved in the other area, when mixed, exceed their solubility level, carbon monoxide gas is liberated. Such gas evolution results in disruption of the process and potentially dangerous conditions. The maximum rate of electrical power input must therefore be restricted to relatively low levels, which, of course, reduces the production rates that can be reached.
In the aforesaid known arrangement the minimum liquid metal level for normal operation is restricted by the requirement that the row of inductors furthest away from the tapping spout of the furnace must always be below the metal level, even when the furnace is tilted farthest to the tapping side. This restriction, and the requirement that the heaps must be formed to completely cover the metal bath, reduces the space available for forming heaps and for combusting the gasses emitted from the burden or fuel that may be introduced to the furnace. Depending on the angle of repose of the heaps, the projected surface area for heat transfer to the heaps is also restricted by the restriction in the minimum liquid metal level.
A further feature of such known arrangement is that both single or double loop inductors are always mounted with their channels parallel to the longitudinal axis of horizontal drum furnaces. This means that the normally oval throat openings have their longitudinal axes parallel to the longitudinal center line of the furnace. Since the inductor throats are usually separated by significant refractory material walls that support the rest of the refractory lining of the furnace, the number of inductors in a row per unit length of furnace is restricted. Hot spots are therefore formed typically 4 to 5 meters apart. This feature further adds to the inhomogeneity of the movement of material in the heaps.
It is an object of this invention to provide an apparatus for the aforesaid purpose with which the aforesaid problems can be overcome or at least minimized.
According to the invention apparatus for a metal reduction and melting process, in which a metal and carbon containing burden is heated in an induction furnace comprising a heating vessel in which the burden can float in at least one heap on a liquid metal bath in the vessel, is provided, characterized in that the apparatus includes at least one induction heater or inductor which is located at or towards the bottom center line of the vessel.
Preferably the furnace comprises a channel type induction furnace.
Further according to the invention such at least one induction heater serves as the only external heating source of the vessel.
Further according to the invention, the vessel is of elongated tubular configuration and includes a plurality of such inductors which are located in a row which extends longitudinally along the bottom center line of the vessel.
Still further according to the invention the vessel includes towards its upper end a plurality of ports through which burden can be loaded into the vessel, the ports being arranged in two spaced apart longitudinally extending rows so that burden loaded through them will extend as two adjacently located heaps floating on the liquid metal bath, the heaps each being of wedge shape configuration in cross section, with the wider end or xe2x80x98apexxe2x80x99 of a heap being located towards the wall of the vessel and the narrower end or xe2x80x98toexe2x80x99 towards the middle of the vessel.
It will be appreciated that with such an arrangement the heaps will be heated directly below their xe2x80x98toesxe2x80x99 and the average velocity of movement of the burden particles perpendicular to the surface of the heaps elsewhere will hence be minimised, so that most of the burden material will be consumed at or near the toes of the heaps (i.e. in the valley formed between the two rows of heaps), and therefore directly above the inductors.
With this arrangement one can accordingly guard against the possibility that the burden particles are either under or over exposed to radiation in the furnace.
Furthermore, because of the central location of the inductors, the liquid level of the metal bath in the vessel, and hence the volume of the liquid metal itself, can be made much lower than what the case is with the aforesaid known arrangements, in this manner preventing the inhomogeneity referred to earlier, and accordingly also giving rise to a reduction in the electrical power requirement.
Further according to the invention the inductors are so mounted that their longitudinal axes extend at substantially right angles relative to the longitudinal axis of the furnace.
It will be appreciated that with such an arrangement more inductors can be installed per unit of length of the vessel, and the number of hot spots formed under the valley between the rows of heaps is increased because the distance between the hot spots is reduced.
Further according to the invention the configuration of the apparatus is such, and the reaction conditions inside the vessel so controlled, that the burden extends in the manner of a bridge over the whole of the liquid metal bath.
Such an arrangement will ensure that substantially all the reduction of the metal takes place in the burden, i.e. in the solid phase.
The said configuration of the apparatus may, for example, relate to the number and/or location of the aforesaid ports through which the burden is loaded into the vessel.
The said control of the reaction conditions, again, may be effected by controlling any one or more of the following:
1. The rate at which burden is supplied to the vessel;
2. The particle size of the burden;
3. The degree of mixing of the metal and carbon containing components of the burden;
4. The rate at which heat is supplied to the vessel by the induction heater(s);
The rate at which heat is generated by any gas(es) and/or other fuels burnt in the vessel in the space above the heaps.
The heat referred to in 5 (above) may, for example, be from burning carbon monoxide escaping from the burden in the vessel with oxygen-, or oxygen/air mixture-, burners located in the vessel in the area above the burden.
The heat formed as a result of such burning, as well as the radiant heat reflected from the roof of the vessel, may be utilized for at least preheating the burden inside and/or outside the vessel.
Further according to the invention the air/or air/oxygen mixture utilized in said burners may contain a finely divided material which can xe2x80x98glowxe2x80x99 at the temperatures resulting form such burning of the carbon monoxide and/or fuel above the heaps.
Such xe2x80x98glowxe2x80x99 improves the radiancy of the flame, thus increasing its heating effect on the burden.
The finely divided material may, for example, comprise soot.
The finely divided material may also include or comprise lime.
Such lime may assist in the removal of sulphur from the gases present in the furnace.
Further according to the invention the vessel includes at least one outlet port for the molten metal and/or slag formed during the reaction.
Still further according to the invention the metal making process comprises a steelmaking process in which a mixture of carbon in the form of finely divided coal or coke, and a suitable iron oxide containing ore in finely divided form, is heated in the vessel to cause the reduction of the iron oxide and the melting of the resulting steel, which can then be tapped as steel containing less than 0.1% carbon.